1. Field of Invention
This invention relates to radar systems and more particularly to improving the accuracy of the determination of the elevation angle of a target.
2. Description of the Prior Art
Phase interferometer radar, because of its high degree of versatility, is particularly suited for use in airborne radar systems. The phase interferometer radar can simultaneously provide angular elevation and azimuth target coverage, and a large span of range coverage at a data rate of less than one second for all of the aforementioned data.
The elevation angle that is subtended by a target is obtained by receiving a target return signal on a dual antenna array and is proportional to the phase difference between the target signals received by each antenna. The dual antenna array provides the target's elevation angle because the antennas are geometrically arranged so that only a target return signal at an elevation angle of zero arrives at both antennas simultaneously, and the time differential of the incidence of the target signals at each antenna is a known function of the elevation angle of the target.
Radar signals are typically in the microwave frequency spectrum, where signal amplification and the determination of phase difference is difficult. For this reason, the target signal received at each antenna of the array is heterodyned with a local oscillator, thereby providing two intermediate frequency signals which still maintain the same phase difference as the originally received target signals. The intermediate frequency signals (on the order of 30 megahertz) are each amplified and applied to the inputs of a phase detector which provides a signal proportional to the phase difference of the target signals and therefore to target elevation angle. However, the circuitry that is used for heterodyning, amplifying and phase detecting may cause an unequal phase shift of each intermediate frequency signal and thereby cause an error in the measurement of the elevation angle of a target.